1. Field of the Invention
The present invention relates to the production of olefins from a feed comprising at least one lower aliphatic alcohol, e.g., methanol, its corresponding ether or mixtures thereof. More particularly, the invention relates to the catalytic conversion of the feed to an olefinic product using large size catalyst particles.
2. Description of Prior Art
The demand for olefinic feedstocks has increased rapidly in the past few years due to the increased need for synthetic fibers, plastics and petrochemicals. The increase in demand for olefinic feedstocks, such as ethylene and propylene, has periodically caused a shortage of these basic raw materials either because of a limitation in petroleum feedstocks of suitable quality or a limitation in the present olefinic production capacity. Thus, alternative sources of ethylene production from non-petroleum sources are required to keep pace with the demand for ethylene and other olefins.
It is now known that feed comprising lower alcohols and/or oxygenates thereof, such as methanol, ethanol, ether, (e.g., dimethyl ether), aldehydes and ketones, can be converted to gasoline grade hydrocarbons or to olefins by contacting the feed with zeolite catalysts.
U.S. Pat. No. 3,894,106 is directed to the production of olefins from aliphatic ethers by catalytic conversion with, for example, an HZSM-5 zeolite catalyst.
U.S. Pat. No. 3,979,472 is directed to the conversion of lower alcohols and their ethers with a composite of antimony oxide and a ZSM-5 zeolite to produce a mixture of ethylene, propylene and mononuclear aromatics.
U.S. Pat. No. 4,025,572 improved the processes for selecting ethylene by diluting a ZSM-5 zeolite with an inert diluent. A similar result is achieved through the use of subatmospheric partial pressure of the feed, according to U.S. Pat. No. 4,025,575.
U.S. Pat. No. 4,025,571 is directed to improved processes for selecting ethylene by employing a ZSM-5 zeolite in a large crystal form of at least about 1 micron.
U.S. Pat. No. 4,148,835 discloses the combination of the large crystal ZSM-5 zeolite and added metals.
The reaction is usually conducted in a fixed bed, in a fixed bed tubular reactor, or in a fluidized bed reactor. If the process, known as methanol-to-gasoline (MTG) or methanol-to-chemicals (MTC) process depending upon the product obtained, is conducted in a fixed bed or a fixed bed tubular reactor, it is usually carried out in two stages. The first stage comprises the conversion of the feed to dimethyl ether (DME) in the DME reactor, and the second stage the conversion of the effluent of the DME reactor to the gasoline-boiling point range hydrocarbons or to chemicals, such as olefins. Both stages of the reaction are carried out in the presence of a catalyst: the first stage with a gamma alumina catalyst (see, e.g., U.S. Pat. No. 3,931,349), and the second stage with an intermediate pore zeolite catalyst, such as, for example, ZSM-5. If the reaction is carried out in a fluidized bed reactor, the entire course of the reaction is catalyzed by an intermediate pore zeolite catalyst (see, e.g., U.S. Pat. No. 3,998,898). The intermediate pore zeolite catalysts used in the process are defined as zeolites which have a silica (SiO.sub.2) to alumina (Al.sub.2 O.sub.3) mole ratio of at least 12, a Constraint Index of about 1 to 12 and a crystal density of at least 1.6 grams per cubic centimeter (g/cm.sup.3).
The conversion reaction of methanol to hydrocarbon products rich in ethylene and other light olefins (MTC process) is known to be autocatalytic on certain zeolite catalysts. This means that the light olefin products initially formed from methanol act as agents accelerating the methanol conversion reaction process. The selectivity of light olefins from methanol in an MTC process is inversely related to methanol partial pressure. Thus, in order to maximize light olefin production, lower MTC reactor operating pressures are desirable.
It is an object of this invention to maximize light olefin conversion from methanol, DME or mixtures of methanol and DME. This and other objects will be apparent in the following description of the subject invention.